Welcome to the Bunnings Workshop community @Danno72. It's fabulous to have you join us, and many thanks for your question.
There are quite a few Keter deck boxes, and the instructions for them can be found on their website. If you click on your version, you'll then find the assembly instructions under the product features label.
I would presume that you're finding it difficult to remove the end piece because these boxes utilise click together connectors. I do not doubt that once they are connected, it would be tricky to undo the panels. If you start at one corner and work your way across with a thin flat-head screwdriver, you might be able to release the connectors one at a time.
What was the reasoning behind needing to remove the end? If you have run into issues during the assembly process, I trust our team in-store would be more than happy to assist.
Please let me know if you need further assistance or have questions